1. Field of the Invention
The present invention relates to a semiconductor memory device and its fabricating method, e.g. a magneto resistive element provided in a magneto resistive random access memory (MRAM) and its peripheral structure.
2. Description of the Related Art
The MRAM is a general term for solid state memories which act as recorded information carriers utilizing the direction of magnetization of ferromagnetic material and which enable recorded information to be rewritten, retained, and read as required.
A memory cell in the MRAM normally has a structure in which a plurality of ferromagnetic materials are stacked together. Information is recorded by using binary information “1” and “0” to represent the relative arrangement of magnetizations of the plurality of ferromagnetic materials in the memory cell, i.e. to indicate whether the directions of the magnetization are parallel or antiparallel with one another. Recording information is written to the memory by using current magnetic fields to reverse the directions of magnetization of ferromagnetic materials of each memory cell.
The MRAM is perfectly nonvolatile and enables information to be rewritten 1015 times or more. Furthermore, the MRAM enables nondestructive reading and does not require any refresh operations. Accordingly, it enables a read cycle to be reduced. It is also resistant to radiations compared to charge accumulation type memory cells. Thus, the MRAM has a large number of advantages in terms of functions compared to conventional semiconductor memories using dielectrics. The degree of integration per unit area of the MRAM and the time required by the MRAM for a write or read are expected to be generally equivalent to those of a DRAM (Dynamic Random Access Memory). Accordingly, the non-volatility of the MRAM, its major characteristic, is expected to be utilized to use it as an external storage device for portable equipment, embedding it in an LSI, or apply it to a main memory of a personal computer.
An MRAM that is now examined so as to be put to practical use uses a magnetic tunnel junction (hereinafter simply referred to as an “MTJ”) for the memory cell. Such an example is described in, for example, “IEEE International Solid-State Circuits Conference 2000 Digest Paper”, TA7.2. The MTJ mainly comprises a three-layer film including a ferromagnetic layer, an insulating layer, and a ferromagnetic layer. A current tunnels the insulating layer. The resistance value of the junction varies in proportion to the cosine of the relative angle between the directions of magnetization in both ferromagnetic metal layers. The resistance value of the junction is largest when the directions of magnetization in both ferromagnetic layers are antiparallel with each other. This is a tunnel magneto resistive effect. One type of MTJ has a structure that retains data utilizing a difference in magnetic coercive force between both ferromagnetic materials. Another type of MTJ has a so-called spin valve structure in which an antiferromagnetic material is arranged adjacent to one of the ferromagnetic materials to pin the directions of magnetization. The spin valve structure aims reduction of write current and improve the magnetic field sensitivity. An MRAM having the spin valve structure is described in, for example, “Japanese Journal of Applied Physics”, 1997, No. 36, p.200.
A brief description will be given of a conventional method of forming an MTJ element having the spin valve structure.
First, a switching transistor is formed on a semiconductor substrate. Subsequently, an interlayer insulating film, a local interconnect layer, a write interconnect layer, and a contact plug are formed in this order. Then, a nonmagnetic conductive film is formed on the interlayer insulating film as a leading interconnect layer.
Next, a ferromagnetic layer is formed on the leading interconnect layer as a pinning layer. Furthermore, an insulating layer is formed on the pinning layer as a tunnel barrier film. Subsequently, a ferromagnetic layer is formed on the tunnel barrier film as a free layer.
Moreover, the free layer, the tunnel barrier film, and the pinning layer are patterned using a photolithography technique and ion milling. This completes an MTJ element.
Next, an SiO2 film is formed on the MTJ element in order to protect the MTJ element. Then, the SiO2 film and the nonmagnetic conductive film are patterned using the photolithography technique and etching. This completes a leading interconnect layer.
Subsequently, an interlayer insulating film is formed so as to cover the MTJ element. Furthermore, a contact plug is formed in the interlayer insulating film so as to reach the free layer.
The MTJ element is formed as described above.
However, in the conventional MRAM, the upper and lower ferromagnetic layers, arranged opposite each other via the tunnel barrier film, may be electrically short-circuited at their ends. Thus, the yield of the MRAM decreases significantly. This is mainly because when a junction is etched using ion milling, residue containing metal remains near the tunnel barrier at a certain probability. The tunnel barrier film has a very small thickness of about 1 to 1.5 nm. That is, the upper and lower substrates are adjacent to each other at a very small distance of 1 to 1.5 nm. Thus, if the residue is larger than 1 to 1.5 nm in size, a short circuit may occur. However, for a large-scale MRAM, it is substantially impossible to avoid this defect. As the degree of integration of the MRAM increases, it tends to become increasingly difficult to obtain acceptable products.
It is contemplated that the above short circuit problem may be solved by, for example, allowing ions to be incident at about 45° during an ion milling step. In this case, the sides of the MTJ are tapered. As a result, the probability of occurrence of a defect is expected to decrease. However, in an MRAM of a Gbit class, an MTJ element has a size of, for example, 0.1×0.2 μm. The distance between adjacent MTJ elements is about 0.1 μm. Then, to avoid an electric short circuit between the adjacent MTJ elements, ions are desirably allowed to enter the substrate surface as perpendicularly to it as possible during the ion milling step. That is, the short circuit between the MTJ elements and the short circuit between the ferromagnetic layers are traded off with each other.